In a conventional electrostatographic printing process a photoconductive element is charged in the dark, typically with a corona, and is then imagewise exposed. Exposure to the light generates positive-negative charge pairs which discharge those areas of the photoconductor that were exposed to the light, thereby forming an electrostatic image on the photoconductor. The image is then developed with a toner and the developed image can be transferred to a receiver, such as paper, to make a copy. After each copy, the photoconductor must be cleaned, recharged, and exposed to the light image again.
While that procedure produces good copies, it can require a great deal of time under certain circumstances. For example, if the image on the photoconductor is written by a laser and is a high resolution image having many millions of pixels, a great deal of time will be required for the image-wise exposure, and, if this exposure must be repeated for each copy, making multiple copies can require an inordinate amount of time. This is especially true if color copies are to be produced because three or four imagewise exposures must be made for each color copy.
In order to speed up the process of making multiple copies, it would be desirable to eliminate the necessity of recharging and imagewise exposing the photoconductor after each copy is made. However, if a conventional photoconductive element is used and a single imagewise exposure is used to make several copies, the quality of the copies quickly deteriorates because the developing and transfer steps disrupt the distribution of charges on the photoconductor.
In order to overcome this problem special photoconductive elements have been constructed that have an insulating layer in between a photoconductive layer and a conductive layer. (See U.S. Pat. No. 4,407,918.) Using this special element, the photoconductor is charged at the same time that it is imagewise exposed. This results in the charges on the exposed areas being buried at the insulating layer while the charges on the unexposed area remain on the surface of the photoconductor. However, because the element is charged at the same time as the imagewise exposure is made, the buried charges are proportionally greater than the charges on the surface of photoconductor, with the result that the overall surface potential of the photoconductor is uniform. The photoconductor is then uniformly exposed which drives the charges on the surface down to the insulating layer. But, since there were fewer charges on the surface, the electrostatic image is preserved at the insulating layer, and because the electrostatic image is buried at the insulating layer, it is preserved for a long time and is not significantly disrupted by the developing or transfer steps. Multiple copies can therefore be made without a significant loss in quality from one copy to the next.
However, it was found that these copies were unsharp at the edges of the images and, while the quality of the copies did not deteriorate, the quality was poorer than the quality obtained using a conventional photoconductive element because the edges of the image were blurred.